1. Field of the Invention
This invention relates to devices having utility in the gauging or measuring of certain properties of threads placed on the end of tubular goods, and more particularly, to a device for measuring the pitch diameter of threaded pipe ends and the like, and for determining the taper of the threads placed in frusto-conical array on the inner or outer sides of the ends of tubular members. The device also has utility in determining the existence in a threaded pipe end of an out-of-round condition.
2. Brief Description of the Prior Art
In the production of oil and gas, a number of situations are encountered in which elongated strings or courses of tubing sections joined in end-to-end relation are utilized for conveying fluids. In many of these situations, it is of considerable importance to proper performance and extended service life of the tubing or pipe string in use that the joints between sections of the pipe or tubing be made up to provide a strong, durable connection without looseness or play. In the assembly of casing strings used to case oil and gas wells, for example, it is of major importance that a tight connection or joint be formed between sections or lengths of casing, particularly at the joints near the upper end of the string where the joint is under the greatest stress due to the great weight of the lower portion of the tubing string which hangs from such upper joints. If the joint is not more than merely hand tight, joint failure or unthreading may occur, and a major portion of the string dropped in the well bore with very serious technical and economic problems resulting. It is also important that a casing string joint not be made up excessively tight by threading, because this results in undesirable galling of the threads.
In order that relatively high precision can be obtained in the making up of casing string joints, as well as in other situations where the threaded ends of two tubular members are to be joined, it is important that the size and configurations of threads used on the ends of such joined sections be standardized, and that the tightening procedures used in effecting joinder also be standardized in correlation to the way in which the standardized threads fit in each other within the joint. To this end, standards specifying the size configuration and orientation of threads on the end of tubular goods used in the oil and gas industry have been specified within that industry, and such standards have been published in the United States in the Specification for Threading, Gauging and Thread Inspection of Casing, Tubing, and Line Pipe Threads, as promulgated by the American Petroleum Institute. This publication sets forth the standard dimensions which will be characteristic of pitch diameter for various types of tubing and casing threads, and for the various outside diametric dimensions of such pipe and tubing. Dimensional figures are also provided which standardize the distance or length from the end of the particular thread-carrying pipe in question to the plane through the threads in which the pitch diameter is mesured. Finally, there is generally specified, where a tapered thread is involved, such as in pin and box connections, a standard dimension of taper which is to characterize the various diametric sizes of such pipe.
Where threaded pipe and tubular goods carry threads which conform to the specified standards, and proper and standard procedures are used in making up the joints by which sections of such pipe and tubular goods are interconnected, no difficulty is normally experienced as a result of failure of the joints, or of shortened service life of the threads carried by the pipe or tubular goods sections so joined by galling of the threads.
To enable a check to be made of the conformity of threads used on casing and other tubing, line pipe and the like to the prescribed standards, the published standards have further provided certain standard gauging procedures, and have made reference to certain types of gauging devices which are advocated for use in measuring the critical dimensions and configurations of threads. It is theorized that by the use of these gauges and gauging procedures, it can be quickly and accurately determined, in a field location, where threaded pipe ends conform to standards and thus can be safely joined, assuming proper joining procedures are utilized, and whether a particular pipe section which carries a female joint or box is properly threaded to facilitate a perfect or at least acceptable threaded joinder to the male or pin end of another section of pipe. Annular ring gauges have been advocated for use in determining the conformity to standards of the pitch diameter, taper and the like of threaded pipe ends. These gauges encircle the threaded pipe end and are moved to a certain axial position along the thread to determine whether the pitch diameter, standoff and other dimensional parameters are standard, or vary from the standard. Certain other types of gauges are used for measuring and determining a cross-sectional configuration of the individual threads, and the proper spacing between crests and troughs of the individual threads.
The types of gauges which have been used in the field for the described purposes have generally, when of the ring gauge type, been time consuming, and to some extent difficult, to use, and have not always provided an accurate indication of the performance capability of the thread being measured. For example, a ring gauge, which capable of measuring and establishing some average diameter which may be interpreted or calculated as the measured pitch diameter, nevertheless is not generally capable of detecting pipe ends which are out-of-round, and which therefore do not have the same diametric dimensions throughout the entire outer peripheral circumference of the pipe. Further gauges currently in use are frequently less than adequate in measuring the factory-coated threads of the box end of a casing section. Such box ends are very frequently damaged in the course of manufacture and subsequent handling by the use of high compressive forces developed by lifting tongs applied to the box end, with the result that distortion of the threads from a truly round or circular configuration occurs, and this anomaly is frequently not detected by the use of the types of gauges currently employed.
A thread gauge which is very useful for gauging pipe threads in the field is illustrated and described in McGaffey U.S. Pat. No. 2,874,475. This instrument can be used for sorting drill pipe to match pin and box ends carried on the pipe so that properly interfitting joints can be realized. Basically, the McGaffey gauge includes an elongated bar having a pair of gauge blocks mounted on the bar, and movable relative to each other. The blocks each carry a pivotally supported gauge arm which can be pivoted to adjust the angle at which the gauge arms extend to each other in a common plane which they occupy.
Although the McGaffey thread gauge can be used for measuring both the taper of a thread course on the end of a pipe, and the end diameter or thread base diameter over a given length of thread, the McGaffey gauge, because of its dependence in use upon abutment of a pipe end against the described base bar, and its dependence upon either end diameter or thread base diameter, does not undertake to gauge the pitch diameter of the thread and thus does not facilitate an accurate determination of the "standoff" between the pin and box in a pin and box joint. The parameter of "standoff" is of substantial importance in determining the tightness of a joint which will be formed between the pin and the joint. The joint tightness is related to the so-called "axial draw." The "diameter draw" is the progression in size of the thread taper which is caused by the longitudinal travel of the interconnected pin and box in undergoing axial draw.